A Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) is an apparatus for elucidating molecular structure by measuring the mass of molecular ions and fragment ions, and has become a basic standard for high-resolution broadband mass spectrometry. The FT-ICR MS is configured to measure the mass of ions in an ion cyclotron resonance (ICR) trap consisting of a cylindrical trap electrode, an activation electrode, a measurement electrode, etc. For example, Korean Patent No. 10-0790532 titled “A method for improving Fourier transform ion cyclotron resonance mass spectrometer signal” discloses a conventional FT-ICR MS.
In the ICR trap, ions exhibit cyclotron motions, including cyclotron rotation, magnetron rotation and axial trapping oscillation. The cyclotron rotation results from the Lorentz force applied on charged ions moving in a static magnetic field. And, the magnetron rotation is induced by the radial electric field gradient formed by an electrostatic trapping voltage in the ICR trap. Further, the ions oscillate linearly along the axial direction of the magnetic field at the trapping oscillation frequency. The FT-ICR MS can measure the mass of ions based on these cyclotron motions of the ions occurring in the ICR trap.
Meanwhile, electron capture dissociation (ECD) refers to a phenomenon in which low energy electrons are introduced to molecular ions to be analyzed to break specific bonds in the FT-ICR MS. ECD is a method of fragmenting ions, which is one of the important processes in elucidation of molecular structure. For example, a process wherein a molecular ion formed from binding of n hydrogen atoms (H) to an atom (M) interacts with an electron to form a fragment ion can be expressed by the following Equation 1.[M+nH]n++e−→[[M+nH](n-1)+]*  [Equation 1]
To induce ECD, electrons are emitted from a cathode of an ECD gun and the emitted electrons are introduced into the ICR trap using, for example, a lens. Fragment ions can be generated in the ICR trap through interaction between the molecular ions and electrons according to Equation 1. Unlike other conventional techniques, ECD can produce fragment ions and is a very important method, for example, in post-translational modification.
However, ECD has the problem that the production efficiency of fragment ions is low. In the FT-ICR MS, the ions are captured using a very high and constant magnetic field for measurement of mass. Under such high magnetic field, electrons are also captured in the magnetic field as the ions are in the magnetic field. Accordingly, an environment is formed in which the electrons are very difficult to move in a direction perpendicular to the magnetic field. ECD occurs when the molecular ions interact with electrons, but, under such an environment, it is not easy for the ions to interact with electrons even when they are close to each other. Due to this problem, the production efficiency of fragment ions using ECD is low.